Recombinational repair protects against the lethal effects of DNA damaging agents. However, recombination can also provide deletions and gene conversion events that contribute to tumorigenesis. Alkylating agents damage DNA and are toxic and carcinogenic. These agents are abundant in our environment and produced in our cells as normal cellular metabolites. Ironically, these agents are also used to treat cancer. 3- Methyladenine (3MeA) DNA glycosylases protect cells from alkylation induced killing by releasing a broad spectrum of damaged bases from the DNA. The goals of this study are to determine how DNA repair influences cellular susceptibility to potentially deleterious recombination events, and to dissect the molecular mechanism of damage induced recombination. Our hypothesis is that DNA normally suppresses recombination by removing recombinogenic lesions, but that repair enzymes can also stimulate recombination if they convert damaged base into a more recombinogenic intermediate. Specific Aim I is to explore the molecular basis for the effects of 3MeA DNA glycosylase activity on homologous recombination in yeast. Specific Aim II is to develop a fluorescence detection system for quantifying the proportions of particular recombination events. This tool will help to reveal how alkyl lesions promote recombination in mammals, and whether or not alkylation damage is likely to underlie spontaneous recombination events in people. Specific Aim III is to apply this detection system to isogenic mammalian cells that carry specific DNA repair defects in order to reveal the role of these pathways in prevent recombination events. The ability of 3 MeA DNA glycosylase activity to protect against homologous recombination in mice will be determined. The mechanisms by which specific alkyl lesions and complex cancer chemotherapeutics stimulate recombination will be explored. The broad long-term objective of this work is to reveal the molecular mechanisms of alkylation damage induced recombination in order to improve our ability to understand the causes of mitotic recombination in people, to identify individuals who have increased susceptibility to cancer, and to improve our ability to develop better anti-cancer therapies.